1) Field of the Invention
The present invention relates to a clamp circuit that converts a video signal to a digital signal and conducts digital signal processing.
2) Description of the Related Art
The picture, quality of the television receiver sets is becoming better and better and the screen size is becoming large and larger day by day. To cope with this progress, there is a tendency to convert an analog video signal to a digital signal and perform some kind of digital signal processing to the digital signal. An analog-digital (A-D) converter is used to perform the conversion. During such conversion, the video signal is clamped in such a manner that the video signal does not exceed a dynamic range of the A-D converter.
While the video signal travels through different components of the circuitry, until it reaches to a digital signal processing circuit, which performs the digital signal processing, a DC level of the video signal undergoes variations. Therefore, even if the video signal is clamped, there is no guarantee that the video signal is securely suppressed at or below a reference level necessary for the digital signal processing.
In other words, by merely clamping the video signal, it is difficult to match the pedestal level. In video signal processing, the pedestal level inserted in the back porch of the horizontal blanking interval becomes the reference of luminance signal processing. When the pedestal level varies, combing or flicker occurs and the image is disturbed. The image with the combing or flicker occurring therein not only is hard to see, but also exerts a bad influence on eyes. Therefore, a method and means that will securely clamp the pedestal level of the video signal is in demand.
For example, in the conventional technology (see, for example, Japanese Patent Application Laid-Open No. 11-88723), the pedestal level is detected from the quantized video signal every field, and detected data are averaged. When a variation of the averaged pedestal level exceeds a predetermined error range, the detected pedestal level is chosen as a new pedestal level. When a variation of the detected pedestal level is within the predetermined error range, the pedestal level detected previously is chosen intactly as the pedestal level. In other words, when the variation of the pedestal level detected every field is within the predetermined error range, the pedestal level is fixed. The video signal is corrected by using a difference between the pedestal level thus determined and a pedestal level of a desired value. Flicker caused by the quantization error in the video signal obtained by converting the analog video signal to the digital signal, i.e., in the quantized video signal is thus suppressed.
When the error range of the pedestal level is made small in the conventional art, however, the pedestal level frequently varies. This results in a problem that occurrence of the combing or flicker cannot be completely prevented.
When the number of times of pedestal level quantization in a field is small, or when the number of bits in quantization is small, the computation error becomes large and the pedestal level frequently varies. This results in a problem that the number of times of sampling must be increased and the number of bits in quantization must be increased. In other words, the average must be calculated with the number of quantization times and the number of bits in quantization increased by using a high-performance A-D converter. This results in a problem that the circuit scale becomes large.